USING FOCUSED ION BEAM MILLING TO INCREASE THE SPATIAL RESOLUTION OF ID-TIMS ZIRCON GEOCHRONOLOGY

Recent research calls to question long-standing ideas about the nature of so-called supereruptions (>500km3 of erupted material) and the links between volcanic and plutonic igneous rocks. The prominent dogma holds that massive magmas are intruded into the shallow earth very quickly, and supereruptions tap only a small fraction of the available magma. However, with the advancement of U-Pb zircon geochronology, geologists now recognize that the assembly of large, underground bodies of magma is generally a prolonged process of magma intrusion, cooling and reheating during subsequent intrusion events, with only small volumes of eruptible magma available at any time.

Chemical zonation in zircon presents an opportunity to understand magma chamber evolution and the links between the chambers and magmas that may erupt from them. Zircon U/Pb geochronology is the most precise method currently available to date igneous rocks. Over the past three decades, it has advanced from dating several milligrams of zircon to single grains by isotope-dilution thermal ionization mass spectrometry (ID-TIMS), and even parts of a grain by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). The spatial resolution offered by LA-ICPMS comes at the expense of analytical precision (~0.1% vs. 2% of the age for TIMS and LA-ICPMS, respectively). I propose combining the precision of ID-TIMS with the spatial resolution of LA-ICPMS using the focused ion beam (FIB) to dissect individual zircon grains prior to ID-TIMS analysis, then analyzing dated samples for rare-earth element (REE) content because REEs are sensitive indicators of magma composition and fractionation.

The Mt. Givens granodiorite of the Sierra Nevada batholith, CA, is often cited as a pluton that could have supported a large super eruption. This possibility is at odds with dating results that suggest the rate of magma accumulation was too low to facilitate eruption, instead favoring pluton assembly. By dissecting the crystal with the FIB and using ID-TIMS to date the sample, and ICP to determine chemical composition, the age and chemical evolution of the single crystal can be studied and used to evaluate hypotheses for the age variation in a sample.